Turning through disorder: Models of bundled mucus strands and microswimmers

With every breath you take, you can inhale dangerous particles. The respiratory system relies on mucociliary transport (MCT) to clear the airways of such particles. This works as follows: deposited particles are captured by a mucus layer lining the airways, and this mucus layer is propelled out of the lungs by the beating action of ciliated cells that collectively create a flow. Mucus consists of mucins which give the suspension elastic properties. In this thesis, we focus on a specific structure, bundled mucus strands, present in the upper airways of large mammals. These strands are created in submucosal glands and can be millimetric in length. Once they are released from the glands, they come together to form large networks, that catch large particles and drag these out of the airways. We devised minimal models by which we could numerically investigate how bundled strands contribute to MCT. Specifically, we were interested in how such strands reorient from an orientation parallel to the direction of the flow, when they just emerge from the gland, to a perpendicular orientation. We studied the role of surface interactions, involving another mucin structure, and local inhomogeneities in the fluid flow. We found that both can drive reorientation, but that surface interactions best fit the experimental observations. We also considered a simple model for a microswimmer in a (model) viscoelastic environment, to see how the motion of such a particle is affected by its surroundings. In connecting to experiments, we found that local contact dynamics are key in capturing its reorientation.